Oxoanions within the meaning the of present invention have the formula XnOm−, XnOm2−, XnOm3−, HXnOm− or H2XnOm2− (where n is an integer 1, 2, 3 or 4, m is an integer 3, 4, 6, 7, or 13, and X is a metal or transition metal of the series Au, Ag, Cu, Si, P, S, Cr, Ti, Te, Se, V, As, Sb, W, Mo, U, Os, Nb, Bi, Pb, Co, Ni, Fe, Mn, Ru, Re, Tc, Al, B, or a nonmetal of the series F, Cl, Br, I, CN, C, N. According to the invention, the expression oxoanions preferably means the formulae XOm2−, XOm3−, HXOm− or H2XOm2−, where m is an integer 3 or 4 and X is a metal or transition metal of the series P, S, Cr, Te, Se, V, As, Sb, W, Mo, Bi, or a nonmetal of the series Cl, Br, I, C, N. Particularly preferably, according to the invention, the expression oxoanions means oxoanions of arsenic in the oxidation states (III) and (V), of antimony in the oxidation states (III) and (V), of sulphur as sulphate, of phosphorus as phosphate, of chromium as chromate, of bismuth as bismuthate, of molybdenum as molybdate, of vanadium as vanadate, of tungsten as tungstate, of selenium as selenate, of tellurium as tellurate or of chlorine as chlorate or perchlorate. Oxoanions which are particularly preferred according to the invention are H2AsO3−, H2AsO4−, HAsO42−, AsO43−, H2SbO3−, H2SbO4−, HSbO42−, SbO43−, SeO42−, ClO3−, ClO4−, BiO42−, SO42−, PO43−. According to the invention, those which are very particularly preferred are the oxoanions H2AsO3−, H2AsO4−, HAsO42− and AsO43− and also SeO42−. Within the meaning of the present invention, the expression oxoanions in the context of the present disclosure also comprises the thioanalogues, where instead of O in the abovementioned formulae, S represents sulphur.
The purity requirements of drinking water have markedly increased in recent decades. Health authorities of numerous countries have developed limiting values for heavy metal concentrations in waters. This relates, in particular, to heavy metals such as arsenic, antimony or chromium.
Under certain conditions, for example arsenic compounds can be extracted from rocks and thus pass into the groundwater. In natural waters, arsenic occurs as an oxidic compound containing trivalent and pentavalent arsenic. In this case it is found that in the pHs prevailing in natural waters the species H3AsO3, H2AsO3−, H2AsO4−, HAsO42− principally occur.
In addition to the chromium, antimony and selenium compounds, readily absorbable arsenic compounds are highly toxic and carcinogenic. However, bismuth passing into the groundwater from ore working is also not safe for health.
In many regions of the USA, India, Bangladesh, China and also in South America, in part very high concentrations of arsenic occur in the groundwater.
Numerous medical studies now confirm that people who are exposed to high arsenic pollution over a long period can develop pathological skin changes (hyperkeratosis) and various types of tumour as a result of chronic arsenic poisoning.
Ion exchangers are widely used for purifying raw waters, wastewaters and aqueous process streams. Ion exchangers are also suitable for removing oxoanions, for example arsenate. For instance, R. Kunin and J. Meyers in Journal of American Chemical Society, Volume 69, page 2874 ff. (1947) describe the exchange of anions, such as, for example, arsenate, using ion exchangers which have primary, secondary and tertiary amino groups.
WO 2004/110623 A1 and EP-A 1 495 800 describe processes for producing iron oxide/iron oxyhydroxide-containing carboxyl-containing ion exchangers. This material adsorbs arsenic down to low residual concentrations, but is limited in its uptake capacity.
EP-A 1 568 660 discloses a process for removing arsenic from water by contacting the water with a strongly basic anion exchanger which contains a specially defined metal ion or a metal-containing ion. EP-A 1 568 660 refers to the fact that the selectivity towards arsenic increases when secondary and tertiary amino groups are converted to quaternary ammonium groups by alkylation, as a result of which strongly basic anion exchangers are characterized according to EP-A 1 568 660. It is of importance that the salt formed from metal and arsenate has a Ksp no greater than 10−5.
In addition, inter alia a process for removing arsenic(III) or arsenic(V) from water by contacting the water with metal-doped ion exchangers is taught in V. Lenoble et al., Science of the Total Environment 326 (2004) 197-207 using manganese dioxide-doped ion exchangers based on polystyrene, in I. Rao et al., Journal of Radioanalytical and Nuclear Chemistry, Vol. 246, No. 3 (2000) 597-600 based on iron(III)-doped chelate resins, and in M. Hruby et al. Collect. Czech. Chem. Commun. Vol. 68, 2003, 2159-2170.
The arsenic adsorbers known from the prior art do not yet exhibit the desired properties with respect to selectivity, capacity and thermal stability. Therefore there is a need for novel ion exchangers or adsorbers in bead form which are specific for oxoanions and/or thioanalogues thereof, in particular oxoanions of arsenic, which are simple to produce, have an improved adsorption of oxoanions and/or thioanalogues thereof, and also display higher thermal stability.
Higher thermal stability of the adsorbers is desirable, since firstly the adsorbers can be stored before use in storage rooms at higher temperatures, or else can come into contact with hot oxoanion-containing water.